[sccp] Implement sending the Inactivity Test on a connection..

Currently this will send a dummy inactivity test message,
there is currently no parsing or API to receive the messages.

The sequencing and credit entries are empty as sequencing
is currently not used at all.

The test is currently limited to send the message and see
if the application is crashing or not.
diff --git a/openbsc/tests/sccp/sccp_test.c b/openbsc/tests/sccp/sccp_test.c
index d3b334f..bd28ed1 100644
--- a/openbsc/tests/sccp/sccp_test.c
+++ b/openbsc/tests/sccp/sccp_test.c
@@ -622,6 +622,7 @@
 
 		printf("\tWriting test data2\n");
 		sccp_connection_write(outgoing_con, test_data2);
+		sccp_connection_send_it(outgoing_con);
 
 		/* closing connection */
 		if (test->close_side == 0)